As a catalyst for purification of exhaust gas, there are widely known three-way catalysts in which a noble metal such as platinum, rhodium or palladium is supported on a metal oxide support made of alumina, titania, silica, zirconia, ceria or the like. Also, it is practiced that multiple species of metal oxide supports are mixed or laminated to utilize the characteristics of the respective metal oxide supports for the purpose of improving catalytic activities. For example, ceria has an oxygen storage capacity (OSC) of storing oxygen when the oxygen concentration in exhaust gas is high and of releasing oxygen when the oxygen concentration in exhaust gas is low; however, its heat resistance is comparatively low. Accordingly, it is practiced that ceria is allowed to form a solid solution or to be mixed with zirconia or alumina, whereby the heat resistance of a catalyst will be improved.
Japanese Unexamined Patent Application Publication No. 2001-170500 (PTL 1) discloses a support prepared by mixing an alumina-based mesoporous powder with a ceria-zirconia solid solution, as well as discloses a catalyst for purification of exhaust gas in which a noble metal is supported on this support. Also, Japanese Unexamined Patent Application Publication No. 2010-298444 (PTL 2) discloses a support comprising an inorganic oxide prepared by mixing boehmite or an alumina powder with a zirconia powder stabilized by Y or Ce, as well as discloses a catalyst for catalytic partial oxidation of hydrocarbons in which an active metal such as a noble metal is supported on this support. Further, Japanese Unexamined Patent Application Publication No. 2006-298759 (PTL 3) discloses a composite oxide, which contains alumina, ceria, and zirconia, prepared by mixing an alumina compound, such as aluminum nitrate or an activated alumina powder, with cerium nitrate or zirconium nitrate, as well as discloses a catalyst for purification of exhaust gas in which a noble metal is impregnated in this composite oxide. In addition, Japanese Unexamined Patent Application Publication No. 2002-160922 (PTL 4) discloses a composite oxide, which contains alumina and a ceria-zirconia solid solution, prepared by the coprecipitation method using an aluminum compound, a cerium compound, and a zirconium compound, as well as discloses a catalyst for purification of exhaust gas in which a noble metal is supported on this composite oxide.
However, the catalysts comprising the supports prepared by the powder mixing methods (PTLs 1-3) or the coprecipitation methods (PTLs 3 and 4) were not necessarily excellent in heat resistance: there were instances where their OSCs or their catalytic activities lower when exposed to high temperatures. Particularly, transition metals such as copper, which are less expensive than the above-described noble metals, are effective as catalysts for purification of exhaust gas; however, they are inferior to the noble metals in heat resistance and are prone to cause grain growth. Thus, there were instances where their catalytic activities lower when exposed to high temperatures.